1. Field of the Invention
The present invention relates to a solid oxide fuel cell.
2. Description of the Related Art
In recent years, there have been proposed a variety of fuel cell assembles by accommodating stacks of fuel cells in the containers to provide energy of the next generation. There have been known a variety of kinds of fuel cells, such as those of the solid high molecular type, those of the phosphoric acid type, those of the molten carbonate type and those of the solid electrolyte type. Among them, the fuel cell of the solid electrolyte type features a high power generating efficiency though its operation temperature is as high as 800 to 1000° C. and offers an advantage of utilizing the waste heat, and its study and development have therefore been forwarded.
The solid oxide fuel cells constituting the fuel cell assemblies can be roughly grouped into those of the cylindrical type and those of the flat plate type. Those of the cylindrical type can be further divided into those having a circular shape in cross section (circular type) and those having a flat elliptic shape in cross section (flat type). The solid oxide fuel cells of the flat type have such advantages as higher output densities than those of the circular type. Their representative example may be the one obtained by providing a solid electrolyte layer and an interconnector on an internal electrode substrate of a flat elliptic shape and providing an external electrode layer on the solid electrolyte layer (see, for example, Japanese Patent No. 2700390 and Japanese Unexamined Patent Publication (Kokai) No. 5-36417).
The fuel cell proposed by the above prior art such as Japanese Patent No. 2700390 is, for example, as illustrated in FIG. 6 wherein an inner electrode substrate 30 having gas flow passages comprises a flat plate 30a and curved portions 30b formed at both ends thereof, and an interconnector 32 is provided on one flat surface of the flat plate 30a. Further, a solid electrolyte layer 34 is laminated on the inner electrode substrate 30 so as to cover a portion where the interconnector 32 has not been provided, and an outer electrode layer 36 is laminated on the solid electrolyte layer 34. As will be understood from FIG. 6, the outer electrode layer 36 is laminated on the solid electrolyte layer 34 so as to surround the other surface of the inner electrode substrate 30 (surface of the side where the interconnector 32 has not been formed) as well as the curved portions 30b. 
In the solid oxide fuel cell of the above structure, curved portions are formed on the inner electrode substrate to prevent the damage at the time of molding and to increase the strength, and the electrode area is increased to increase the output density. According to the study by the present inventors, however, it has been learned that portions of the outer electrode layer 36 located on the curved portions 30b of the inner electrode substrate 30 are facing the interconnector 32 and do not effectively work as an electrode and do not contribute to enhancing the output density. Namely, in these portions, a current path is lengthened between the outer electrode layer 36 and the interconnector 32, and it is considered that the voltage drops to a large extent. In the portions of the outer electrode layer 36 located on the curved portions 30b, further, it is difficult to uniform the thickness thereof and, hence, dispersion occurs in the characteristics.